The MMPP was invented at Defence Research Development Canada (DRDC) Atlantic in response to a need for high power, depth insensitive and wideband underwater sound projection from a single small device. By reversing the orientation of the endcaps of the Axial Drive Resonant Pipe Projector (ADRPP) and with computer optimization, the very wideband MMPP was developed. Since the MMPP only entrains air within its very stiff drive motor, it can operate to great depth without significant changes to its performance. As for bandwidth, some examples of the MMPP have greater than 3 octaves of useable response and therefore can be used in a variety of applications. They are also useful in sonobuoys, torpedo countermeasures, acoustic hammers, mine recognition sonars, underwater positioning systems, and underwater loudspeakers for swimming pools. MMPP's are currently being used in underwater communications system and as acoustic modems and have shown utility as calibration sound sources.
An example of a multi-mode pipe projector (MMPP) is described in U.S. Pat. No. 6,584,039 B1 and illustrated in FIG. 1A. The MMPP has a pair of spaced apart end plates 2 with a twelve ring ceramic stack piezoelectric driver 4 positioned between and coupled to the end plates 2. The driver 4 has a smaller cross-section than the end plates 2. Tubular pipe waveguides 6 having open ends are attached to the end plates 2 and arranged to face each other. The driver 4 is sealed in a neoprene boot 8.
Current multi-mode pipe projector (MMPP) transducers have limited low frequency capability due to the low strain nature of their piezoceramic drive motors and their small size as compared to wavelength of operation. The Terfenol-D magnetostrictive version of the MMPP can operate at a lower frequency than others; however it has limited high frequency capability due to a lack of drive motor breathing modes inherent in piezoceramically driven MMPP's and due to high frequency eddy current-induced losses. Terfenol-D is a magnetostrictive alloy made of terbium, dysprosium and iron metals.
FIG. 1B shows a typical mid-frequency MMPP. The size of such a compact device is approximately 4 inches in diameter and 7 inches long. In order to expand the bandwidth of the MMPP and provide extreme bandwidth coverage with low frequency extension, multiple transducers are required, which take up more space than a single transducer.